The answer is "c", "Gypsum, CaSO₄ • 2 H₂O".
Because in option A,B and D there is a component which acts as a base and reacts with the acid like CO₃²⁻ in limestone, CaCO₃, OH⁻ in slaked lime, Ca(OH)₂ and OH⁻ in hydroxyapatite, Ca₅(OH)(PO₄)₃.
However in Gypsum, CaSO₄ • 2 H₂O, SO₄²⁻ are weak bases as they are the conjugate bases of strong acid H₂SO₄ .
Thus option C shows minimum tendency of reaction with HCl and hence minimum solubility.